it could be an area thing if you're indoors or in a place that's far from a tower with 4G antennas. if you go into settings -> wireless & networks, there should be an option somewhere in there to switch between 1x/3g (no LTE), and all networks (1x/3G/LTE). rebooting the phone could sometimes help, along with checking if the 4G phones at a verizon store are actually connected to 4G.
if you think it's your phone, just get it swapped out with another.
edit: according to another employee post on here:
Press Wireless & Networks
Press Mobile Networks
Press Network Mode
Make sure the setting is set to LTE/CDMA"